3GPP long term evolution (LTE) adopts orthogonal frequency division multiple access (OFDMA) as a downlink communication scheme. In a radio communication system to which 3GPP LTE is applied, a base station transmits a synchronization signal (synchronization channel: SCH) and a broadcast signal (broadcast channel: BCH) using predetermined communication resources. A terminal first secures synchronization with the base station by catching an SCH. Then, the terminal acquires parameters (e.g. frequency bandwidth) specific to the base station by reading BCH information (see Non-Patent Literatures 1, 2, and 3).
Furthermore, after completing the acquisition of the parameters specific to the base station, the terminal transmits a connection request to the base station and establishes communication with the base station. The base station transmits control information to the terminal with which communication is established through physical downlink control channel (PDCCH) as necessary.
The terminal then makes a “blind decision” on each of a plurality of pieces of control information included in the received PDCCH signal. That is, the control information includes a cyclic redundancy check (CRC) portion, and this CRC portion is masked with a terminal ID of a transmission target terminal in the base station. Therefore, the terminal cannot decide whether or not the control information is directed to its own terminal until the CRC portion of the received control information is demasked with the terminal ID of its own terminal. In the blind decision, when a demasking result represents that a CRC calculation is OK, it is determined that the control information is directed to its own terminal.
Furthermore, in 3GPP LTE, automatic repeat request (ARQ) is applied to downlink data from a base station to a terminal. That is, the terminal feeds back a response signal indicating an error detection result of downlink data to the base station. The terminal performs a CRC on the downlink data, and feeds back acknowledgment (ACK) when CRC=OK (no error) and negative acknowledgment (NACK) when CRC=NG (error) to the base station as a response signal. An uplink control channel such as a physical uplink control channel (PUCCH) is used for feedback of the response signal (that is, ACK/NACK signal).
However, since each terminal makes a blind decision on downlink assignment control information directed to its own terminal in each subframe (transmission unit time), the terminal is not always successful in receiving downlink assignment control information. When the terminal fails to receive downlink assignment control information directed to its own terminal in a certain downlink unit band, the terminal even cannot know whether or not downlink data directed to its own terminal exists in the downlink unit band. Therefore, when the terminal fails to receive downlink assignment control information in a certain downlink unit band, the terminal does not generate any response signal to the downlink data in the downlink unit band either. This error case is defined as DTX (discontinuous transmission) of ACK/NACK signals) of the response signal in the sense that transmission of the response signal is not performed on the terminal side.
Here, the control information transmitted from the base station includes resource assignment information including resource information and the like assigned from the base station to the terminal. The PDCCH is used for transmission of this control information as described above. The PDCCH is configured with one or more L1/L2 control channels (L1/L2 CCHs). Each L1/L2 CCH is configured with one or more control channel elements (CCEs). That is, a CCE is a base unit for mapping control information to a PDCCH. Furthermore, when one L1/L2 CCH is configured with a plurality of CCEs, a plurality of consecutive CCEs are assigned to the L1/L2 CCH. The base station assigns an L1/L2 CCH to a resource assignment target terminal according to the number of CCEs necessary for reporting control information to the resource assignment target terminal. The base station then transmits the control information mapped to a physical resource corresponding to the CCE of the L1/L2 CCH.
Here, each CCE has a one-to-one correspondence with a component resource of the PUCCH. Therefore, the terminal that has received the L1/L2 CCH can implicitly specify a component resource of the PUCCH corresponding to the CCEs configuring the L1/L2 CCH, and transmits a response signal to the base station using the specified resource. This allows downlink communication resources to be used efficiently.
As illustrated in FIG. 1, a plurality of response signals transmitted from a plurality of terminals are spread by a Zero Auto-correlation (ZAC) sequence having a Zero Auto-correlation characteristic, a Walsh sequence, and a discrete Fourier transform (DFT) sequence on a time axis, and code-multiplexed within the PUCCH. In FIG. 1, (W0, W1, W2, W3) represents a Walsh sequence having a sequence length of 4, and (F0, F1, F2) represents a DFT sequence having a sequence length of 3. As illustrated in FIG. 1, in the terminal, a response signal of ACK or NACK is first primary-spread within one SC-FDMA symbol on a frequency axis by a ZAC sequence (having a sequence length of 12). Next, the response signal subjected to the primary spreading is associated with W0 to W3 and F0 to F2 respectively and subjected to inverse fast Fourier transform (IFFT). The response signal spread by the ZAC sequence having a sequence length of 12 on the frequency axis is transformed into a ZAC sequence having a sequence length of 12 on the time axis by the IFFT. That is the signal subjected to the IFFT is further subjected to processing equivalent to secondary spreading using a Walsh sequence (sequence length of 4) and a DFT sequence (sequence length of 3).
Furthermore, the standardization of 3GPP LTE-advanced that realizes faster communication than 3GPP LTE has started. A 3GPP LTE-advanced system (which may also be hereinafter referred to as “LTE-A system”) follows the 3GPP LTE system (which may also be hereinafter referred to as “LTE system”). In order to realize a downlink transmission rate of a maximum of 1 Gbps or above, 3GPP LTE-advanced is expected to introduce base stations and terminals capable of performing communication at a wideband frequency of 40 MHz or above.
In an LTE-A system, in order to simultaneously realize communication at an ultra-high transmission rate several times as fast as a transmission rate in the LTE system and backward compatibility with the LTE system, a band for the LTE-A system is divided into “unit bands” of 20 MHz or less, which is a support bandwidth for the LTE system. That is, the “unit band” herein is a band having a width of maximum 20 MHz and defined as a base unit of a communication band. Furthermore, a “unit band” in a downlink (hereinafter, referred to as “downlink unit band”) may be defined as a band divided by downlink frequency band information included in the BCH broadcasted from the base station, or a band defined by a dispersive width when the downlink control channel (PDCCH) is dispersed and arranged in the frequency domain. Furthermore, a “unit band” in an uplink (hereinafter, referred to as “uplink unit band”) may be defined as a band divided by uplink frequency band information included in the BCH broadcasted from the base station, or as a base unit of communication band of 20 MHz or less, which includes physical uplink shared channel (PUSCH) region near the center thereof and PUCCHs for the LTE at both ends thereof.
Furthermore, in 3GPP LTE-Advanced, the “unit band” may also be expressed as “component carrier(s)” in English. Furthermore, the “unit band” may also be defined by a physical cell number and carrier frequency number, and may be called “cell.”
The LTE-A system supports communication using a band that bundles several unit bands, so-called “carrier aggregation.” In the LTE-A system, carrier aggregation in which the number of unit bands set for an arbitrary terminal supporting LTE-A system is (hereinafter referred to as “LTE-A terminal”) is the same between the uplink and the downlink, so-called “symmetric carrier aggregation” and carrier aggregation in which the number of unit bands set for an arbitrary LTE-A terminal is different between the uplink and the downlink, so-called “asymmetric carrier aggregation” are being under study. The asymmetric carrier aggregation is useful when throughput requirements for an uplink are different from throughput requirements for a downlink. Furthermore, cases are also expected to be supported where the number of unit bands is asymmetric between the uplink and the downlink, and different unit bands have different frequency bandwidths.
FIG. 2 is a diagram illustrating asymmetric carrier aggregation applied to individual terminals and a control sequence thereof. FIG. 2 illustrates an example in which a bandwidth and the number of unit bands are symmetric between an uplink and a downlink in a base station.
In FIG. 2, a setting (configuration) is made for terminal 1 such that carrier aggregation is performed using two downlink unit bands and one uplink unit band on the left side, whereas a setting is made for terminal 2 such that although the two same downlink unit bands as those in terminal 1 are used, an uplink unit band on the right side is used for uplink communication.
Focusing attention on terminal 1, signals are transmitted/received between an LTE-A base station and an LTE-A terminal configuring an LTE-A system according to a sequence diagram illustrated in FIG. 2A. As illustrated in FIG. 2A, (1) terminal 1 is synchronized with the downlink unit band on the left side when communication with the base station starts, and reads information of the uplink unit band which forms a pair with the downlink unit band on the left side from a broadcast signal called “system information block type 2 (SIB2).” (2) Using this uplink unit band, terminal 1 starts communication with the base station by transmitting, for example, a connection request to the base station. (3) Upon deciding that a plurality of downlink unit bands need to be assigned to the terminal, the base station instructs the terminal to add a downlink unit band. In this case, however, the number of uplink unit bands does not increase, and terminal 1 which is an individual terminal starts asymmetric carrier aggregation.
Furthermore, in the LTE-A system, studies are being carried out on activation/de-activation for each downlink CC on a downlink unit band set for a terminal using signaling (e.g., reporting using MAC header) faster than signaling used for the setting (RRC signaling). FIG. 3 shows a conceptual diagram of this activation and de-activation.
FIG. 3 shows an example of a case where a downlink bandwidth of an LTE-A system managed by a base station is 100 MHz and each downlink unit band has 20 MHz. FIG. 3 illustrates a ease where the base station sets a downlink band of 60 MHz to terminal 1 (that is, sets (configures) three downlink unit bands) and activates two out of the three downlink unit bands. The base station activates one or a plurality of downlink unit bands out of the three unit bands set for terminal 1 as necessary, and can thereby make a flexible communication speed setting with the terminal. However, downlink CC#b is a specific downlink unit band always activated for terminal 1. The specific downlink unit band that the base station always activates for terminal 1 may be called “anchor component carrier” (anchor CC) or “primary component carrier (PCC)”. On the other hand, downlink unit bands other than the anchor downlink unit band (PCC) may be called “secondary component carriers (SCCs).” Furthermore, the above-described specific downlink unit band may be defined as “downlink unit band used by the terminal to establish initial communication” in FIG. 2.
Similarly, FIG. 3 illustrates a case where the base station sets an 80 MHz downlink band (that is, sets (configures) four downlink unit bands) for terminal 2 and activates two out of the four downlink unit bands. The base station activates one or a plurality of downlink unit bands out of the four unit bands set for terminal 2, and can thereby make a flexible communication rate setting with terminal 2 as required.
Through this activation/de-activation, the terminal needs only to monitor necessary downlink unit bands when necessary, and it is thereby possible to obtains an effect of reducing power consumption of the terminal.